The present invention relates to cellular telecommunication systems, and more particularly to the automatic allocation of frequency channels to cells in a cellular telephone system.
In cellular telephone networks, maintaining or improving the speech quality in each communication channel is of great importance. One factor affecting speech quality is the level of co-channel interference. Co-channel interference results when two cells, located close to one another geographically, use the same frequency. One way to avoid this problem is to assign a dedicated group of frequency channels to each cell in the network, so that no two cells utilize the same frequency channel. While this would clearly avoid the problem of co-channel interference, the network would quickly run out of frequency channels since there are only a fixed number of frequency channels available.
To avoid running out of available frequency channels, cellular telephone networks employ reuse plans. Reuse plans allow a network to assign a frequency channel to more than one cell. While some co-channel interference is expected, excessive co-channel interference can be avoided by making sure the two or more frequency channels are only allocated in cells that are spaced sufficiently far enough apart.
In general, reuse plans are well known to those skilled in the art. A fixed reuse plan, as the term suggests, involves the assignment of a fixed, dedicated group of frequency channels to each cell in the network. Frequency channels can be assigned to more than one cell as long as the cells are located far enough apart to avoid excessive co-channel interference.
As previously mentioned, each cell in a network that employs a fixed frequency channel reuse plan will be limited to the specific frequency channels assigned; therefore, the traffic-handling capability for each cell will be limited despite the avoidance of excessive co-channel interference. In other words, fixed reuse plans are inherently inflexible; there is no provision to adjust the frequency channel allocations in each cell as demand fluctuates from cell to cell over the course of a given time period. The result is a degradation in both speech quality and traffic-handling capacity. Therefore, adaptive reuse plans, also known as adaptive or dynamic channel allocation plans, were conceived.
Adaptive frequency channel reuse plans attempt to avoid the degradation in speech quality and traffic-handling capability by providing greater flexibility. Rather than assigning a fixed group of frequency channels to each cell in the network, allocations will vary over time to meet the changing needs of each cell. The way this is accomplished is by periodically measuring the signal quality for each frequency channel in each cell. Techniques for measuring signal quality include using dedicated received signal strength transceivers (RSSI) and evaluating the bit error rate (BER) of received signals. As required, cells will have frequency channels allocated as long as the signal quality measurements for the channels meet or exceed certain signal quality criteria. For example, if cell A requires an additional frequency channel to handle an increase in telephone traffic, frequency channel X is not likely to be allocated if it is already being used in a nearby cell. Co-channel interference due to the use of frequency channel X in the nearby cell will be measured in cell A as interference. Thus, frequency channel X will not meet the required signal quality criteria.
There are different types of adaptive channel allocation plans. The primary difference between each plan is the application of criteria used to determine whether a frequency channel should or should not be allocated in a given cell at a given time. For example, H. Eriksson, "Capacity Improvement by Adaptive Channel Allocation", IEEE Global Telecomm. Conf., pp. 1355-1359, Nov. 28-Dec. 1, 1988, suggests using the mobiles to measure the signal quality of the downlink for each channel, then channels are assigned on the basis of those having the highest carrier to interference (C/I) ratios. A somewhat different approach is expressed by G. Riva, "Performance Analysis of an Improved Dynamic Channel Allocation Scheme for Cellular Mobile 74 Radio Systems", 42nd IEEE Veh. Tech. Conf., pp. 794-797, Denver 1992, where frequency channels may be allocated if their signal quality measurements meet or exceed a preset C/I threshold. In Y. Furuya et al., "Channel Segregation, A Distributed Adaptive Channel Allocation Scheme for Mobile Communication Signals", Second Nordic Seminar on Digital Land Mobile Radio Communication, pp. 311-315, Stockholm, Oct. 14-16, 1986, an adaptive channel allocation plan is described whereby the recent history of previously measured signal quality for each channel is used in making channel allocation decisions.
When employing a conventional adaptive channel allocation plan, it is most effective to measure both uplink (i.e., the radio path from the mobile to the base station) and downlink (i.e., the radio path from the base station to the mobile) signal quality for each frequency channel. In digital systems such as D-AMPS (Digital Advanced Mobile Phone System), uplink measurements can be made by equipment located in the base station. Downlink measurements can be made by the mobile assisted handover (MAHO) unit in each mobile; the mobile then transmits the measurements back to the base station.
While adaptive channel allocation strategies provide a more flexible plan which ultimately leads to better signal quality and traffic-handling capacity, the criteria by which particular frequency channels are selected for allocation to a given cell have not yet been refined to consider all of the various system parameters impacted by the allocation process. For example, combiners, commonly used in cellular base stations to combine signals from several frequency channels for coupling to a base station's antenna, typically include a plurality of resonator filters each of which is tuned to a particular frequency associated with a radio channel on which the base station is to transmit. However, to avoid difficulties in tuning each resonator filter to the desired frequency, it is important to provide an excluded bandwidth around the desired frequency so that the resonator filter does not also pick up signal energy from a nearby frequency to which another resonator filter in the combiner is tuned. For example, it is common to provide an exclusionary bandwidth of up to 630 kHz around each frequency to which a resonator filter is tuned, with the size of the exclusionary bandwidth being determined based upon, for example, the frequency range and output power for transmission. This means that, in practice, a buffer of up to about 10 radio frequency channels to either side of a selected frequency will be set aside as an exclusionary bandwidth and not allocated for use by that base station.
This characteristic of combiners, however, is not taken into account in conventional dynamic frequency allocation techniques. Accordingly, as will be illustrated in more detail below, these conventional techniques do not necessarily make efficient use of the bandwidth available to each base station. Therefore, the invention presents a technique for frequency packing which takes into account the any desired frequency separation or excluded bandwidth, e.g., due to the operation of combiners, to more efficiently use the limited bandwidth available to each base station and each cell.